Plant Biotechnology and Genetics: Principles, Techniques and Applications

6.4 Translation

How does the information in the mRNA result in the synthesis of a polypeptide? Multiple
cellular players are involved in the synthesis of a polypeptide. First, the structure of a poly-
peptide needs to be understood. Polypeptides are made up of a linear sequence of amino
acids. There are 20 common types of amino acids (Table 6.1), and to form a polypeptide,
amino acids are joined together in a chain by peptide bonds (Fig. 6.11). Proteins can be
composed of either a single polypeptide or multiple polypeptide chains that are the same
or different in amino acid sequence (Fig. 6.11).
Once the mRNA is transported out of the nucleus, it must be properly “read” ortrans-
latedbyribosomesin order to produce a polypeptide. But how many nucleotides of the
mRNA are needed to code for one amino acid? Three consecutive nucleotides, called a
codon, are required to be read to specify one amino acid. This code isnonoverlapping,
meaning that once a triplet is read, the cellular machinery reads the next three nucleotides
and so on in linear fashion. Therefore, within a givenreading frame(there are three possible
reading frames; see Fig. 6.12), a nucleotide cannot be present in more than one codon.
Since there are four nucleotide possibilities (A, G, C, or U) at each of the three codon pos-
itions, there are 4
4
4 ¼64 different combinations or codons (Table 6.2). A codon is
written in the 5^0! 30 direction as it would be read on the mRNA molecule. Since there are
more codons than amino acids, some codons actually specify the same amino acid, and so
the code is considered to be degenerate in that regard. Three codons (UAA, UAG, and
UGA) do not code for any amino acid. These arestop codons, and when any one is
read, it signals the cellular machinery to stop translation.

TABLE 6.1. The 20 Amino Acids Commonly Found in Proteins

Amino Acid

Three-Letter

Abbreviation

One-Letter

Abbreviation

Alanine Ala A

Arginine Arg R

Asparagine Asn N

Aspartate Asp D

Cysteine Cys C

Glutamine Gln Q

Glutamate Glu E

Glycine Gly G

Histidine His H

Isoleucine Ile I

Leucine Leu L

Lysine Lys K

Methionine Met M

Phenylalanine Phe F

Proline Pro P

Serine Ser S

Threonine Thr T

Tryptophan Try W

Tyrosine Tyr Y

Valine Val V

148 MOLECULAR GENETICS OF GENE EXPRESSION